1. Field of the Invention
This invention relates to a laser piercing method capable of satisfactorily forming a hole serving as a starting point in shape-cutting a workpiece.
2. Description of Related Art
In laser cutting, a workpiece is cut by repetitive physical reaction in which a focal point of a laser beam having a high energy density is set in a prescribed position in a thicknesswise direction of a workpiece and then irradiated upon the workpiece, and the position of the focal point is then moved along a targeted cutting line, thereby, a base material at the focal point and proximal to the focal point is instantaneously melted and removed by evaporation. Accordingly, a desired cut can be performed without restricting the material of the workpiece.
For example, when using a steel plate as a workpiece, the focal point of the laser beam is set to a position slightly inward from the surface of the steel plate in a thicknesswise direction, and the laser beam having the focal point as a starting point is then irradiated to the steel plate, so that a surface side of the steel plate and a minute portion in a thicknesswise direction are instantaneously melted and vaporized substantially at the same time.
The steel plate, accordingly, has a recess or a groove formed thereto by the vaporization of the base material. The workpiece can therefore be cut by moving the position of the focal point along a targeted cutting line, in which the base material is continuingly vaporized in association with the irradiation of the laser beam.
However, the vapor of the base material accumulated in the portion irradiated by the laser beam may prevent energy from being efficiently applied upon the workpiece. It is therefore general to inject an assist gas along with the laser beam, so that the injection energy thereof can remove the vapor of the base material.
Particularly when the workpiece is a ferrous metal in which burning reaction of the base material is expected, injection of oxygen gas which is injected along with the laser beam burns a portion of the base material and simultaneously removes the burnt material with the injection energy thereof to enable removing and cutting a wide area base material with the focal point of the workpiece serving as a starting point.
Shape-cutting, which is aimed to cut a workpiece into various shapes, is generally performed by first forming a pierced hole on a portion of the workpiece other than a portion thereof targeted as a product (piercing) and then cutting the workpiece with the hole serving as a starting point.
In such case, a laser torch is arranged in a position facing a portion of the workpiece targeted for forming the hole, a laser beam is then irradiated from the foregoing position to melt a portion of the base material of the workpiece, and an assist gas further removes the melted base material; thereby, a recess formed on a surface of the workpiece is grown in a depthwise direction for forming the penetrated hole.
In piercing the workpiece for forming a penetrated hole in a thicknesswise direction, the melted base material is removed from the surface of the workpiece by the injected energy of the oxygen gas until a hole is penetrated.
In this situation, the melted base material scatters and adheres to the distal end of the laser torch or enters into the laser torch creating a possibility of damaging the laser torch. It is therefore general to prevent the melted material from adhering to the laser torch by elevating the laser torch without changing the position of the laser condensing lens.
Nevertheless, although the foregoing conventional method of piercing can reduce the melted material from adhering to the laser torch, the melted material cannot be removed efficiently from the recess formed in the workpiece since the position of the focal point is fixed. Therefore, such inefficient removal of the melted material raises problems as requiring more time for piercing and causing increase in cost.
It is an object of this invention to solve the foregoing problems by providing a laser piercing method capable of reducing the time required for piercing a workpiece and satisfactorily forming a pierced hole.
The laser piercing method of this invention for solving the foregoing problems provides a piercing method for piercing a workpiece by irradiating a laser beam to a workpiece and injecting an assist gas to the workpiece, in which a piercing process is performed by setting the level of a nozzle during piercing execution to a value higher than the level of the nozzle during cutting execution and setting the pressure of the assist gas during piercing execution to a value higher than the pressure of the assist gas during cutting execution, and a cutting process, subsequent to the piercing process, is performed by setting the level of the nozzle and the pressure of the assist gas to a value prescribed for cutting execution, respectively.
With the foregoing laser piercing method, raising the level of the nozzle prevents melted material, such as spatter, from adherently scattering during piercing execution. Further, increasing the pressure of the assist gas prevents reduction of, for example, flow amount, flow rate, or purity of the assist gas, even when the level of the nozzle is raised.
Another laser piercing method of this invention provides a piercing method for piercing a workpiece by irradiating a laser beam to a workpiece and injecting an assist gas to the workpiece, in which a piercing process is performed by setting the level of a focal point of the laser beam during piercing execution lower than the level of a focal point of the laser beam during cutting execution, and a cutting process, subsequent to the piercing process, is performed by setting the level of the focal point of the laser beam to a value prescribed for cutting execution.
With the foregoing laser piercing method, lowering the level of the focal point of the laser beam during piercing execution enables a concentrated portion of laser energy to work deeply upon a workpiece in a thickness direction so as to melt and vaporize a portion of a base material of the workpiece and allow assist gas to remove the melted material and vaporized material of the base material from the surface of the workpiece; thereby, piercing can be performed satisfactorily and the time required for piercing can be shortened to restrain cost increase.
Further, when a pulsed laser output of the laser beam during piercing execution is step by step increased in a plurality of steps, in which each step is comprised of a set of plural pulses having a uniform output, the piercing process is initially performed with a uniform and low laser output and is step by step performed with a uniform and higher laser output being increased in association with the growth of a hole; thereby, piercing can be performed more satisfactorily compared to the conventional process of continuously increasing the pulsed laser output.
Further, the time required for piercing can be shortened to restrain cost increase when the pulsed laser output frequency of a laser beam during piercing execution is set no less than 20 Hz and no more than 200 Hz.
Another preferable laser piercing method provides a piercing method for piercing a workpiece by irradiating a laser beam to a workpiece and injecting an assist gas to the workpiece, in which the piercing process is performed by setting the pressure of the assist gas during piercing execution to a value higher than the pressure of the assist gas during cutting execution and increasing or reducing the pressure of the assist gas in relation to the lapse of time, and the cutting process, subsequent to the piercing process, is performed by setting the pressure of the assist gas to a value prescribed for cutting execution.
With this preferable laser piercing method of increasing or reducing the pressure of the assist gas, the assist gas being increased or reduced in relation to the lapse of time can effectively remove melted material and vaporized material of a portion of a base material of a workpiece, which has been melted or vaporized by laser energy, from the surface of the base material of the workpiece. Accordingly, piercing can be performed in a satisfactory state and the time required for piercing can be shortened to restrain cost increase.